The present invention relates to photographic printing and, more particularly, is directed to an improved vacuum frame.
One of the most useful tools in the graphic arts industry is the vacuum contact frame which is used for making contact photographic prints. The vacuum frame should be distinguished from the vacuum platen or easel, which is used in cameras or enlargers, to hold large sheets of film at the focal plane of the camera lens or in focus. In contact printing, a transparent mask sheet is placed in intimate contact with a sheet of light sensitive material.
In the two major graphic arts applications, the light sensitive material may be photographic film or photo sensitive printing plates. For this discussion, the photosensitive medium will be referred to as a film. The mask may contain a positive or negative image, depending on whether the photo sensitive material responds in a positive or negative way to light (e.g. some photo printing plates are positive responding). During the exposure, light passes through the mask and onto light sensitive film. Where the mask is opaque, the light is blocked and the underlying photo film remains unexposed. By this means, the pattern of the mask is transferred to the photo film.
In order to retain sharpness during the transfer exposure, it is necessary for the mask and photo film to be in intimate contact. If the mask and film sheets are merely pressed together, bubbles of air will remain between them and the resulting uncontrolled separation will cause local blurring and loss of definition of the transferred image.
Vacuum frames have been in use for many years to help prevent this problem. In the vacuum frame, the mask and film are placed under a glass top plate and are supported on the bottom by a flexible pressure blanket. The edges of the blanket are sealed against the glass plate in such a way that the glass and blanket together, form an air-tight chamber. The air may then be drawn out of the resulting glass/blanket chamber. This evacuation of air from the chamber causes the external air at ambient pressure to force the blanket up against the mask/film sandwich. The force of the blanket against the film helps force out the air from between the film sheets. A vacuum pump draws off this air so that, once full contact between the sheets is made, the contact will remain for the duration of the evacuation.
There are two significant problems with this arrangement. The first is that it takes substantial time to withdraw the air from the space between the film sheets, after the volume defined by the blanket has been evacuated. A second problem is that under strong vacuum, the high pressure of the blanket tends to press the edges of the film sheets together so as to form a seal which traps a residuum of air between the sheets, thus creating bubbles or pockets in which air is trapped. These bubbles keep portions of the mask and photo film separated, which results in blurred images. Moreover, the bubbles are not spatially stabilized and may "wander" around during the course of a long duration exposure.
Early attempts to alleviate the problem of trapped air bubbles involved pressing inflated bladders against the back of the mask/photo material sandwich. No attempt was made, in these early versions, to evacuate the sandwich region. An example is the photographic blueprint apparatus which is the subject of U.S. Pat. No. 628,347 to McDade. This device consists of a simple frame with a glass top in which a single chambered, hand inflated bladder presses against the back of the photo medium sandwich.
A patent from a slightly later time, U.S. Pat. No. 725,969 to Jarney, shows a mechanically pumped bladder arrangement which also includes a roller that is designed to squeeze out residual trapped bubbles of air.
More recently, U.S. Pat. No. 3,995,955, to Topfer, teaches the use of a simple inflatable backing bladder, in conjunction with a vacuum frame, to squeeze out remaining bubbles of air and speed the evacuation process. It should be noted that the combination fails to regulate or monitor the air flow.
U.S. Pat. No. 3,510,217 to Cirimele et al. discusses the problem, in Column 1, Lines 44-56 thereof, of small air gaps or bubbles being formed between the photoconductive plate and the charge retentive surface of the electrographic paper, which prevents formation of a charge image on those portions of the paper which are coextensive with the bubbles. In accordance with the invention therein, which is used in an electrographic camera, the photoconductive plate is supported from a glass plate, and a supply roll of electrographic recording paper is provided, with a strip thereof being disposed overlaying the photoconductive plate. The electrographic paper comprises a thin film of dielectric coated on a conductive paper backing and which forms a charge retentive surface. In accordance with the invention, a soft elastic pressure pad assembly is provided which is formed by a soft elastic pad of sponge rubber and a thin flexible skin of non-porous rubber thereover. The pad has a dome-shaped surface which abuts the paper and which is flattened when pushed against the photoconductive plate.
As stated at Column 3, Lines 12-22, the purpose of this dome-shape is to permit the pressure pad to squeeze the air, in the spaces between the paper and the photoconductive plate, outwardly from the center of the plate towards its edge as the pad is pressed against the paper and the plate. This prevents air bubbles from being trapped between the paper and the plate. Such air bubbles prevent a nominal contact between the paper and the plate, and thus, prevent uniformity in the charge transfer to the charge retentive surface of the electrographic paper. The pad is further provided with diagonally crossed slots for further assisting in squeezing air out of the spaces between the paper and the plate.
FIG. 4 of the Patent shows an alternative pressure pad assembly comprised of a flat dome-shaped bag made of rubber and filled with a fluid such as air or water. The operation and purpose of this latter embodiment is identical to that of the pressure pad, that is, for squeezing air from the center towards the ends between the two sheets.
U.S. Pat. No. 3,007,390 to Forester et al. discloses an arrangement similar to U.S. Pat. No. 3,510,217, having particular applicability to a vacuum frame. The blanket is mounted under tension so that contact with the superimposed sheet is made first at the center of the vacuum frame, then moves progressively outwardly with an ironing effect so as to eliminate the formation of bubbles and wrinkles.
U.S. Pat. No. 4,087,181 to Alexander et al. discloses an arrangement providing a glass plate, a thin flexible template positioned thereunder and a composite two layer strip of radiation sensitive material positioned therebetween. A pad of soft resilient flexible material having an upper pressure surface which is angled downwardly from a center pressure ridge is provided. In the inoperative position (FIG. 2), the ridge on the pad is spaced from the strip of material positioned below the template. A plunger assembly moves toward the plate and biases the ridge against the center of the strip material. Subsequent movement of the plunger assembly (FIG. 4) causes the material of the pad to be displaced so that contact between the pressure surface and the strip material increases outwardly to sweep air from between the layers of the strip material.
See also U.S. Pat. Nos. 263,242; 2,139,956; 3,547,537; 3,955,163; and 4,484,813 for different arrangements.
Recently, a vacuum frame has been marketed which uses rollers on the backside of the pressure blanket that move from one edge of the blanket to the other. The purpose of the rollers is to progressively squeeze the air from one edge of the film sandwich to the other. This frame is manufactured by the Theimer Company of Germany. When operating correctly, no air bubbles are left behind in the film sandwich and with such intimate contact it is theoretically possible to obtain sharp, controllable exposures. In practice, however, this roller arrangement has two significant defects. Some bubbles of air still remain, and the combination is slow to operate, requiring substantial set up time before an exposure can be made. This is a significant drawback in, for example, a production environment where time is an important factor.
What has been needed is a new way to extract all of the air from the film sandwich so that no bubbles are left behind. Moreover, the new way should be sufficiently gentle not to impose great pressure on the glass top plate. Further, if the evacuation process can be speeded, the productive efficiency of the vacuum frame will be much improved.
In many of the above-described conventional vacuum frames, a vacuum frame blanket is drawn up against the under surface of a transparent plate of glass as the air is evacuated from between these two elements. A ring of beading material, normally fastened to the periphery of the vacuum blanket forms a seal when this ring is pressed up against the under surface of the glass top plate. The result is an air tight chamber with a rigid glass plate being the upper wall and the flexible blanket acting as the lower wall. Resting on top of the vacuum blanket, and below the glass plate, is a sandwich consisting of a mask sheet which carries a transparency image and a sheet of photosensitive material. The photosensitive material, typically, is either ordinary photographic film or a metal plate which will be later used as a lithographic printing plate. In principle, when the air is evacuated from between the blanket and the glass plate, the mask/film sandwich is pressed firmly against the glass plate. The air is squeezed out from between the mask and the film and the two are placed in such intimate contact that passage of light through the mask will transfer the mask's image to the photographic emulsion.
This is the theory of the vacuum frame. In practice, however, it turns out that the traditional vacuum frame design has substantial deficiencies. The great problem of the conventional design is that the air flow out of the vacuum chamber region between the glass plate and the flexible blanket is not uniform or controlled. It happens that different parts of the vacuum blanket make contact with the glass plate at different times. This causes barriers to form in different parts of the vacuum frame chamber which impede the flow of air, with a consequent slowing of the evacuation process. What is worse, the conventional vacuum frame chamber has a tendency to seal together the edges of the mask/film sandwich before all of the air has been removed from between these two sheets. This means that a thin, uncontrolled, bubble of air is often left between the mask and the film. This bubble significantly degrades the quality of the image transfer during the exposure.
In order to overcome the problems associated with the aforementioned vacuum frames, the Applicant herein invented a novel photographic vacuum contact printing frame, which is the subject of U.S. Pat. No. 4,504,142, the entire disclosure of which is incorporated herein by reference.
This frame has the advantage that it can successfully evacuate the air from between a masking sheet and an unexposed sheet of photosensitive material, in much shorter time than previous frame designs. It also provides a much higher degree of precision in the photographic exposure since the evacuation is much better.
Specifically, the apparatus disclosed therein replaces roller assemblies and single chamber bladders with a progressively inflated, multi-chambered bladder. The bladder is divided into a plurality of interconnected and inflatable chambers. In the preferred embodiment, the chambers are constructed in such a way as to resemble flat pockets which inflate into low ceilinged cavities.
Pressurized air is introduced to a single, innermost chamber, thereby inflating this chamber first. The innermost chamber is surrounded by a nested arrangement of additional chambers.
As the central chamber inflates, air bleeds to the next adjacent chamber, causing its inflation sometime after the inflation of the first chamber. As each chamber of the arrangement is inflated, the inflation of the next chamber begins. This arrangement applies pressure first to a small, central area, and then progressively, to regions closer to the edges of the film sandwich. This radial, "peristaltic" pressure wave gently squeezes the air towards all the edges of the film sandwich in such a way that no air bubbles in the film sandwich are left behind.
A further advantage accrues from the arrangement. Because of the efficiency of the peristaltic motion, the air can be squeezed from between the sheets of film at a much faster rate than heretofore has been possible. This means that the vacuum frame, assisted by the pressurized blanket, can come to full stabilized vacuum in a much shorter period of time with resulting improvements in productivity.
In accordance with the different embodiments, the squeezing may occur starting from a corner, an edge or the middle of the two sheets. The nested bladders or partitions are interconnected by apertures therein so that after the innermost bladder is filled, the air escapes to the next most outer bladder and so on.
This Patent teaches that, in one preferred embodiment, the innermost chamber, and all of the other inner chambers of the nesting arrangement, are offset to one side of the exposure area. This is because the mask/film sandwich is normally held in lateral alignment by a system of pins inserted through holes punched along one edge of the mask and photo film. In order to maintain best alignment, it is important that the pin edge of the film sandwich be pressed together first by the vacuum frame, with the residual air being gently squeezed from this edge towards the far edges of the sandwich. For this reason, the nested chamber arrangement should be skewed towards the pin edge of the exposure area.
However, this preferred embodiment of the Patent has some significant deficiencies. Primary among these is that, during the evacuation process, the air is evacuated first from a non-critical portion of the vacuum frame. Only after this region is cleared, does air from the important part of the frame leave the frame's interior. Moreover, the critical air must flow the length of the vacuum blanket after the blanket is substantially drawn up against the glass top plate. The blanket, of course, contains grooved channels to make this possible. However, the narrowness of the air passage grooves means that the evacuation of the critical air takes much longer than is needed.
Specifically, with this arrangement, the chamber evacuation port was deliberately located at the foot of the nested bladders because it was thought that, as the bladders progressively inflated, they would naturally squeeze the air in the vacuum chamber directly toward the evacuation port. In practice, the squeezing action does occur, but before this squeezing action can take full effect, the air being drawn from the vacuum chamber has already caused the blanket to seal up against the glass in the vicinity of the evacuation port. This creates a barrier at the foot of the inflatable assembly which impedes the evacuation of the air from the head region. This barrier leaves a pocket of air near the head of the mask/film sandwich which takes a significant length of time to evacuate through the grooves in the vacuum blanket. Adjustments to the timing of the inflation and evacuation sequence somewhat moderate the problem, but it is evident that the chamber evacuation is not as swift as it might be.
Further, it is becoming increasingly common for retractable pin type boards to be used to register the mask and film while they are in a vacuum frame. With the frame manufactured and sold according to the Patent, the pin board is placed on top of the blanket in the vacuum frame so that the pins are located at the head of the bladder assembly. It is desirable that the head of the assembly be first evacuated so that accurate alignment of the mask and film assembly will first be obtained. However, due to the aforementioned problem where an air pocket is left near the head of the mask/film sandwich, such alignment at the pins may not occur first.
Another deficiency resulted from the placement of the air bleed holes which communicate from each bladder to the next bladder in the nested sequence. If these holes are not properly placed, they tend to seal up against the fabric of the surrounding bladder and cause a catastrophic failure of the system. Indeed, on several occasions the glass top plate was shattered because of improper inflation of the bladder assembly. Other deficiencies had to do with the materials, and ease and reliability of construction. The deficiencies will be discussed hereinafter.